MASS SPECTRUM...........BENZENE

In mass spectrometry, a substance is bombarded with an electron beam having sufficient energy to fragment the molecule. The positive fragments which are produced (cations and radical cations) are accelerated in a vacuum through a magnetic field and are sorted on the basis of mass-to-charge ratio. Since the bulk of the ions produced in the mass spectrometer carry a unit positive charge, the value m/e is equivalent to the molecular weight of the fragment. The analysis of mass spectroscopy information involves the re-assembling of fragments, working backwards to generate the original molecule. A schematic representation of a mass spectrometer is shown below:

A very low concentration of sample molecules is allowed to leak into the ionization chamber (which is under a very high vacuum) where they are bombarded by a high-energy electron beam. The molecules fragment and the positive ions produced are accelerated through a charged array into an analyzing tube. The path of the charged molecules is bent by an applied magnetic field. Ions having low mass (low momentum) will be deflected most by this field and will collide with the walls of the analyzer. Likewise, high momentum ions will not be deflected enough and will also collide with the analyzer wall. Ions having the proper mass-to-charge ratio, however, will follow the path of the analyzer, exit through the slit and collide with the Collector. This generates an electric current, which is then amplified and detected. By varying the strength of the magnetic field, the mass-to-charge ratio which is analyzed can be continuously varied.
The output of the mass spectrometer shows a plot of relative intensity vs the mass-to-charge ratio (m/e). The most intense peak in the spectrum is termed the base peak and all others are reported relative to it's intensity. The peaks themselves are typically very sharp, and are often simply represented as vertical lines.
The process of fragmentation follows simple and predictable chemical pathways and the ions which are formed will reflect the most stable cations and radical cations which that molecule can form. The highest molecular weight peak observed in a spectrum will typically represent the parent molecule, minus an electron, and is termed the molecular ion (M+). Generally, small peaks are also observed above the calculated molecular weight due to the natural isotopic abundance of ¹³C, ²H, etc. Many molecules with especially labile protons do not display molecular ions; an example of this is alcohols, where the highest molecular weight peak occurs at m/e one less than the molecular ion (m-1). Fragments can be identified by their mass-to-charge ratio, but it is often more informative to identify them by the mass which has been lost. That is, loss of a methyl group will generate a peak at m-15; loss of an ethyl, m-29, etc.
The mass spectrum of toluene (methyl benzene) is shown below. The spectrum displays a strong molecular ion at m/e = 92, small m+1 and m+2 peaks, a base peak at m/e = 91 and an assortment of minor peaks m/e = 65 and below.

The molecular ion, again, represents loss of an electron and the peaks above the molecular ion are due to isotopic abundance. The base peak in toluene is due to loss of a hydrogen atom to form the relatively stable benzyl cation. This is thought to undergo rearrangement to form the very stable tropylium cation, and this strong peak at m/e = 91 is a hallmark of compounds containing a benzyl unit. The minor peak at m/e = 65 represents loss of neutral acetylene from the tropylium ion and the minor peaks below this arise from more complex fragmentation.
















Basics 
 

Mass spectrometry is based on slightly different principles to the other spectroscopic methods.The physics behind mass spectrometry is that a charged particle passing through a magnetic field is deflected along a circular path on a radius that is proportional to the mass to charge ratio, m/e. In an electron impact mass spectrometer, a high energy beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion. If the molecular ion is too unstable then it can fragment to give other smaller ions. The collection of ions is then focused into a beam and accelerated into the magnetic field and deflected along circular paths according to the masses of the ions. By adjusting the magnetic field, the ions can be focused on the detector and recorded.

• Probably the most useful information you should be able to obtain from a MS spectrum is the molecular weight of the sample.
• This will often be the heaviest ion observed from the sample provided this ion is stable enough to be observed.

Terminology 
 

Molecular ion	The ion obtained by the loss of an electron from the molecule
Base peak	The most intense peak in the MS, assigned 100% intensity
M+	Symbol often given to the molecular ion
Radical cation	+ve charged species with an odd number of electrons
Fragment ions	Lighter cations formed by the decomposition of the molecular ion. These often correspond to stable carbcations.

Spectra

The MS of a typical hydrocarbon, n-decane is shown below. The molecular ion is seen as a small peak at m/z = 142.  Notice the series ions detected that correspond to fragments that differ by 14 mass units, formed by the cleaving of bonds at successive -CH₂- units

The MS of benzyl alcohol is shown below. The molecular ion is seen at m/z = 108.  Fragmentation via loss of 17 (-OH) gives a common fragment seen for alkyl benzenes at m/z = 91.  Loss of 31 (-CH₂OH) from the molecular ion gives 77 corresponding to the phenyl cation. Note the small peaks at 109 and 110 which correspond to the presence of small amounts of 13C in the sample (which has about 1% natural abundance).

Isotope patterns

• Mass spectrometers are capable of separating and detecting individual ions even those that only differ by a single atomic mass unit.
• As a  result molecules containing different isotopes can be distinguished.
• This is most apparent when atoms such as bromine or chlorine are present (⁷⁹Br : ⁸¹Br, intensity 1:1 and ³⁵Cl : ³⁷Cl, intensity 3:1) where peaks at "M" and "M+2" are obtained.
• The intensity ratios in the isotope patterns are due to the natural abundance of the isotopes.
• "M+1" peaks are seen due the the presence of ¹³C in the sample.

The following two mass spectra show examples of haloalkanes with characteristic isotope patterns.

The first MS is of 2-chloropropane. Note the isotope pattern at 78 and 80 that represent the M amd M+2 in a 3:1 ratio.
Loss of ³⁵Cl from 78 or ³⁷Cl from 80 gives the base peak a m/z = 43, corresponding to the secondary propyl cation. Note that the peaks at m/z = 63 and 65 still contain Cl and therefore also show the 3:1 isotope pattern.
 

 

The second MS is of 1-bromopropane. Note the isotope pattern at 122 and 124 that represent the M amd M+2 in a 1:1 ratio. Loss of ⁷⁹Br from 122 or ⁸¹Br from 124 gives the base peak a m/z = 43, corresponding to the propyl cation. Note that other peaks, such as those at m/z = 107 and 109 still contain Br and therefore also show the 1:1 isotope pattern.